Project Summary/Abstract Early mortality in pulmonary arterial hypertension (PAH) is due, in part, to fibrotic remodeling of pulmonary arterioles that increases vascular resistance, although effective therapies for vascular fibrosis in PAH do not exist currently. Overlap among established signaling pathways that regulate collagen function in reparative (e.g., dermal wound healing) and pathogenic (i.e., causing end-organ damage) fibrosis suggests that reductionist methods are limited for identifying key mechanisms mediating vascular fibrosis in PAH. In this NIH Research Project Grant Program proposal, we use systems biology to develop a protein-protein interactome (fibrosome) and analyzed genes in silico according to their association with dermal wound healing or vascular fibrosis. The pro-oxidant hormone aldosterone (ALDO) promotes collagen synthesis in dermal wound healing. However, ALDO also increases collagen in pulmonary artery endothelial cells (PAECs) in vitro to induce vascular fibrosis in PAH in vivo. Thus, we propose that segregating ALDO-regulated genes in the fibrosome according to their association with pathogenic or adaptive fibrosis will allow novel molecular targets responsible for arterial fibrillar collagen synthesis in PAH to emerge. From our network analysis, we identified targeting of the Cas-L protein NEDD9 by ALDO as a critical pathway delineating pathogenic from reparative fibrosis. We have demonstrated previously that oxidation of functionally essential protein cysteinyl thiols by ALDO is associated with adverse pulmonary vascular remodeling. In the current proposal, we used computational modeling to predict Cys18 in the NEDD9 protein-docking region as a likely redox sensitive cysteinyl thiol. Oxidative modification of NEDD9 Cys18 by ALDO prevented binding of NEDD9 with Smad3, which is required for normal NEDD9 degradation. Impaired NEDD9-Smad3 binding, in turn, was associated with increased NEDD9 and NEDD9-dependent fibrillar collagen levels in ALDO-treated PAECs in vitro, but not in cell types involved in reparative fibrosis. ALDO treatment of PAECs also induced NEDD9 in co-cultured pulmonary artery smooth muscle cells in vitro, and NEDD9 correlated with fibrosis burden in remodeled pulmonary arterioles from PAH patients and in experimental PAH models characterized by hyperALDO in vivo. Therefore, the central hypothesis of the current proposal is that oxidation of NEDD9 Cys18 by ALDO prevents Smad3- dependent degradation of NEDD9, which results in NEDD9 accumulation in PAECs in vitro to promote pathogenic pulmonary vascular fibrosis and PAH in vivo. The study Aims are: (1) use mass spectrometry and Raman spectroscopy to demonstrate that NEDD9 Cys18 oxidation by ALDO in PAECs is an essential molecular mechanism underlying pulmonary vascular collagen synthesis in vitro, and (2) use genetic ablation and gain-of- function methods, including NEDD9-/- mice, to demonstrate that NEDD9 is a critical regulator of pulmonary vascular fibrosis and pulmonary hypertension in vivo. These studies aim to identify redox regulation of NEDD9 as a novel treatment target for PAH and other diseases characterized by pathogenic vascular fibrosis.